Question: Initially there were 66 cars parked in the hardware store parking lot. During the following 3 hours some more cars parked at the hardware store, and now there are 81 cars in the parking lot. No cars left the parking lot during this time. How many cars parked in the hardware store parking lot after the initial 66 cars?
Explanation: The number of cars that came and parked at the store is the difference between the number that were parked earlier and the number that are parked now. The difference is $81 - 66$ cars. $81 - 66 = 15$ cars.